at what point , after adding , can one say they have "successfully" kept a fish ?

Depends upon the fish. Anyone who successfully keeps a fish has kept it near (or more) than its usual natural lifespan. JMO.
 
id say a year. i have lost fish after a few months due to stressful aggressive tank inhabitants. other because they were lackidasical feeder. i guess it takes a while to starve to death if u eat a little but not enuf. but once they hit the one year mark, only a mistake or an accident witll kill them. i recently lost all of my fish of three years due to a heater malfunction. now i will be buying a controller.
 
I would suggest a different approach. Instead of age, I gauage it by death. If it dies of disease, starving, etc then you have not successfully kept it. If it dies of accidental rock avalanche, general stupidity (IE, accidental overdosing, etc), jumped, etc and it was in good health, then you kept it. If it dies of old age and was very healthy, I would say that you have successfully kept it.
 
Oh, and by "old age" I'm talking more than just a "long time". More like close to its natural life span proportional to its maximum size.

Examples:
Five people buy one small Yellow Watchman Goby about 1" long. Person one has it die in six months because of starvation: that's unsuccessul. Person two has it die in one year because of an avalanche: this is just a moderate because it could have lived longer, but it might not have. Person three has kept it three years and it's still alive: that's successful. Person four has one for one year, bought a second one, had them breed, and was able to raise the fry to adulthood, and the original is still alive: that is a complete success. Person five has had theirs for ten years, has reached full size, and still alive: another complete success.
 
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